1. Disable hibernation (it will remove a file the size of your RAM located in c:\, so between 4gB and 16+gB) by opening a command prompt (press start/Windows key, type 'cmd', then press enter to open it) and typing 'powercfg -h off' then enter.
2. My 'softwaredistribution' folder (in c:\windows\) was 4 gB. Not cool! That folder is for Windows Update. You can delete it (it will be called .old) after you do run the following in the command prompt:
net stop wuauserv
rename c:\windows\SoftwareDistribution softwaredistribution.old
net start wuauserv
3. Use Revo Uninstaller to not just uninstall programs, but remove files and registry keys normally left behind by the regular uninstaller.
4. Run ccleaner (if you've never done this it will probably get rid of 2+gB alone)
5. My MSOCache folder (in c:\) was 500mB. I deleted it and don't seem to have any problems with word/excel (actually, they seem to load much faster).
6. My Installer folder (in c:\windows) was 3.6gB! unfortunately, I couldn't get this down much, but you might be able to (folks on the internet were able to free up a lot). This folder contains uninstall information so the contents are necessary, but there may be extra ("orphaned") information in there which you can remove by downloading msicuu2.exe and running 'msizap G!' in the command line after navigating to the installation folder ('windows installer clean up' in program files)
7. My C:\Windows\ServiceProfiles\NetworkService\AppData\Local\Microsoft\Media Player folder was 600mB, mostly of album art... I deleted it immediately (who uses WMP, anyway?).
I stopped here, because it cleared up about 5gB :)
If you need more, grab a DockBox.